Old tree, leafy on both sides (7)
I believe the answer is:
elderly
'old' is the definition.
(similar in meaning)
'tree leafy on both sides' is the wordplay.
'tree' becomes 'elder' (elder is a kind of tree).
'on both sides' suggests removing the centre (the left and right sides of the word are used).
'leafy' with its centre removed is 'ly'.
'elder'+'ly'='ELDERLY'
(Other definitions for elderly that I've seen before include "Getting on a bit" , "Quite old" , "over the hill" , "Fairly old, past middle age" , "Old or ageing" .)